The present invention relates generally to the fabrication of nanotubes, and in particular to methods of fabricating nanotube structures from an array of catalyst islands on a semiconductor surface.
Carbon nanotubes are recently discovered, hollow graphite tubules. When isolated, individual nanotubes are useful for making microscopic electrical, mechanical, or electromechanical devices. Obtaining individual, high quality, single-walled nanotubes has proven to be a difficult task, however. Existing methods for the production of nanotubes, including arc-discharge and laser ablation techniques, yield bulk materials with tangled nanotubes. The nanotubes in the bulk materials are mostly in bundled forms. These tangled nanotubes are extremely difficult to purify, isolate, manipulate, and use as discrete elements for making functional devices.
One conventional method for producing carbon nanotubes is disclosed in U.S. Pat. No. 5,482,601 issued to Oshima et al. on Jan. 9, 1996. The nanotubes are produced by successively repositioning a rod-like, carbon anode relative to a cathode surface such that a tip of the anode successively faces different portions of the cathode surface. A direct current voltage is impressed between the tip of the anode and the cathode surface so that an arc discharge occurs with the simultaneous formation of carbonaceous deposits containing carbon nanotubes on the cathode surface. The carbonaceous deposits are scraped and collected.
U.S. Pat. No. 5,500,200 issued to Mandeville et al. on Mar. 19, 1996 discloses a method for the bulk production of multi-walled tubes. According to the method, a catalyst is prepared using particles of fumed alumina with an average particle size of about 100 xc3x85. Iron acetylacetonate is deposited on the alumina particles, and the resultant catalyst particles are heated in a hydrogen/ethylene atmosphere. The catalyst particles are preferably reacted with the hydrogen/ethylene mixture for about 0.5 hours in a reactor tube, after which the reactor tube is allowed to cool to room temperature under a flow of argon. Harvesting of the carbon tubes so produced showed a yield greater than 30 times the weight of the iron in the catalyst particles.
Although the methods described by Oshima and Mandeville are effective for producing bulk amounts of carbon tubes or carbon fibrils, the resulting bulk materials are xe2x80x9chairballsxe2x80x9d containing tangled and kinked tubes which one collects into vials or containers. These bulk materials are useful to put into polymers or metals to make composites that exhibit improved properties of the polymers or metals. For making functional microscopic devices, however, these bulk materials are nearly useless because it is nearly impossible to isolate one individual tube from the tangled material, manipulate the tube, and construct a functional device using that one tube. Also, many of the tubes have molecular-level structural defects which results in weaker tubes with poor electrical characteristics.
Atomic force microscopes (AFMs) sometimes employ nanotubes as the scanning tip because nanotubes are resilient and have an atomically sharp tip. However, the manufacturing of nanotube-tipped AFM devices is problematic because the nanotubes must be painstakingly separated from disorganized bundles of nanotubes and attached to the AFM cantilever. It would be an advance in the art of atomic force microscopy to provide a nanotube-tipped AFM device that is simpler to manufacture.
In view of the above, it is an object of the present invention to provide a method for the large scale synthesis of individual distinct single-walled nanotubes. In particular, it is an object of the present invention to provide such a method which allows nanotube growth to be confined to desired locations so that the nanotubes can be easily addressed and integrated into structures to obtain functional microscopic devices. It is a further object of the invention to provide a method for integrating the nanotubes into semiconductor microstructures to obtain a variety of nanotube devices. Further, it is an object of the present invention to provide a nanotube-tipped atomic force microscope device which is simple to manufacture.
These and other objects and advantages will become more apparent after consideration of the ensuing description and the accompanying drawings.
These objects and advantages are provided by an apparatus including a substrate and a catalyst island disposed on the substrate. The catalyst island includes a catalyst particle that is capable of growing carbon nanotubes when exposed to a hydrocarbon gas at elevated temperatures. A carbon nanotube extends from the catalyst particle. The nanotube may be in contact with a top surface of the substrate.
The substrate may be made of silicon, alumina, quartz, silicon oxide or silicon nitride. The nanotube may be a single-walled nanotube. The catalyst may include Fe2O3 or other catalyst materials including molybdenum, cobalt, nickel, or zinc and oxides thereof (iron molybdenum, and ruthenium oxides are preferred). The catalyst island is preferably about 1-5 microns in size.
The present invention also includes an apparatus having a substrate with two catalyst islands and a nanotube extending between the islands. The nanotube provides an electrical connection between the islands, which are electrically conductive. Conductive lines can provide electrical connections to the islands and nanotube. The nanotube may be freestanding above the substrate. A freestanding nanotube can be used as a high frequency, high-Q resonator.
Alternatively, one of the islands is replaced with a metal pad that does not have catalytic properties.
The present invention also includes an atomic force microscopy apparatus that has a catalyst particle disposed on a free end of a cantilever. A nanotube extends from the catalyst particle. The nanotube can be used as the scanning tip of the atomic force microscope apparatus.
The present invention also includes a method of making individually distinct nanotubes on a substrate surface. The method begins with disposing catalyst islands on the surface of a substrate. Then, the catalyst islands are contacted with methane gas at elevated temperature. The nanotubes grown are separate and extend over the surface of the substrate. The separate and distinct nanotubes can be incorporated into microelectronic or microelectromechanical devices.